1. Field of the Invention
The present invention relates to methods of making comb-teeth electrode pairs in a micro oscillating element which includes a rotationally displaceable oscillating portion. The electrode pair can be a pair of comb-teeth electrodes as part of a drive mechanism which drives the oscillating portion, or a pair of comb-teeth electrodes as part of a detection mechanism for detecting the amount of rotational displacement of the oscillating element.
2. Description of the Related Art
In recent years, efforts have been made for making practical use of elements which have a micro-structure formed by micromachining technology. In the field of optical communications technology for example, micromirror elements which are tiny elements capable of reflecting light are gathering attention. In the filed of sensing, attention is paid to tiny acceleration sensors capable of detecting acceleration as well as tiny angular speed sensors capable of detecting angular speed.
In the optical communications, optical fibers serve as a medium through which optical signals are passed. When the optical signal passing through a given optical fiber is switched to another optical fiber, so-called optical switching devices are used in general. In order to achieve high quality optical communications, the optical switching device must have such characteristics as high capacity, high speed and high reliability, in switching action. In view of these, micromirror elements manufactured by utilizing micromachining technology are gathering attention as a switching element to be incorporated in the optical switching devices. The micromirror elements enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous in achieving the above-mentioned characteristics. Micromachining technologies are disclosed in the following Patent Documents 1 through 3 for example.
Patent Document 1: JP-A-10-190007
Patent Document 2: JP-A-10-270714
Patent Document 3: JP-A-2000-31502
FIG. 31 is a partially non-illustrated exploded perspective view of a conventional micromirror element X6 manufactured by micromachining technology. The micromirror element X6 includes: a mirror support 61 which has an upper surface provided with a mirror surface 64; a frame 62 (partially non-illustrated); and a pair of torsion bars 63 connecting these. The mirror support 61 has a pair of ends formed with a pair of comb-teeth electrodes 61a, 61b. The frame 62 is formed with a pair of inwardly extended comb-teeth electrodes 62a, 62b correspondingly to the comb-teeth electrode 61a, 61b. The torsion bars 63 provide an axis for oscillating action of the mirror support 61 with respect to the frame 62.
According to the micromirror element X6 which has the structure as described, a set of comb-teeth electrodes which are placed closely to each other for generation of electrostatic force, e.g. the comb-teeth electrodes 61a, 62a, are apart from each other, making an upper and a lower steps as shown in FIG. 32A when no voltage is applied. When a predetermined voltage is applied on the other hand, as shown in FIG. 32(b), the comb-teeth electrode 61a is drawn in between the comb-teeth electrode 62a while rotationally displacing the mirror support 61. More specifically, when the comb-teeth electrodes 61a, 62a are supplied with a predetermined voltage and whereby the comb-teeth electrode 61a is positively charged and the comb-teeth electrode 62a is negatively charged, then there is static attraction developed between the comb-teeth electrodes 61a, 62a, which causes the mirror support 61 to make a rotational displacement around the axis A6 while twisting the torsion bars 63. By utilizing such an oscillating motion of the mirror support 61, it is possible to switch directions in which light is reflected by the mirror surface 64 on the mirror support.
FIG. 33 shows a manufacturing method for the micromirror element X6. In FIG. 33, views of a section are given to illustrate a process of forming those components which are shown in FIG. 31, i.e. part of the mirror support 61, the frame 62, the torsion bars 63, part of a pair of comb-teeth electrodes 61a, 62a, and part of a pair of comb-teeth electrodes 61b, 62b. The section represents a section of a material substrate (a wafer) to which the manufacturing processes is performed, and more specifically a section of a single block from which a single micromirror element is formed. The section includes sections of a plurality of component regions, and the sectional views are illustrative sequential depictions.
In the method of manufacturing the micromirror element X6, first, a mask pattern 604 is formed on a material substrate 600 as shown in FIG. 33(a). The material substrate 600 is a so called SOI (Silicon on Insulator) wafer, which has a laminate structure including a silicon layer 601 and a silicon layer 602, and an insulation layer 603 between them. The mask pattern 604 has a pattern for masking predetermined regions (including the comb-teeth electrodes 61a, 61b) on the micromirror element X6. The mask pattern 604 is formed by first forming a film of a predetermined mask material on the silicon layer 601, and then patterning the film.
In the manufacture of the micromirror element X6, next, a mask pattern 605 is formed on the silicon layer 602 as shown in FIG. 33(b). The mask pattern 605 has a pattern for masking predetermined regions (including the comb-teeth electrodes 62a, 62b) on the micromirror element X6. The mask pattern 605 is formed by first forming a film of a predetermined mask material on the silicon layer 602, and then patterning the film, while positioning the pattern to the mask pattern 604 which is on the silicon layer 601.
Next, as shown in FIG. 33(c), an anisotropic etching process is performed to the silicon layer 601 via the mask pattern 604, whereby formation is made for structures (the mirror support 61, part of the frame 62, the torsion bars 63, and the comb-teeth electrodes 61a, 61b) which are due on the silicon layer 601.
Next, as shown in FIG. 33(d), an anisotropic etching process is performed to the silicon layer 602 via the mask pattern 605, whereby formation is made for structures (part of the frame 62, and the comb-teeth electrodes 62a, 62b) which are due on the silicon layer 602.
Next, as shown in FIG. 33(e), isotropic etching is performed to the insulation layer 603 to remove exposed portions of the insulation layer 603. The above described process yields the mirror support 61, the frame 62, the torsion bars 63, the comb-teeth electrodes 61a, 62a, and the comb-teeth electrodes 61b, 62b. 
According to the conventional method, as has been described above while making reference to FIG. 33(b), pattern formation in the mask pattern 605 must be made while positioning the pattern with respect to the mask pattern 604. However, it is difficult to do this positioning highly accurately because the mask pattern 604 is patterned on the silicon layer 601 in the material substrate 600 whereas the mask pattern 605 must be patterned on the silicon layer 602 which is on the side away from the silicon layer 601. According to the above-described convention, the mask pattern 604 includes portions for masking the comb-teeth electrodes 61a, 61b, and the mask pattern 605 includes portions for masking the comb-teeth electrodes 62a, 62b. Yet, because it is difficult to pattern the mask pattern 605 highly precisely at a position with respect to the mask pattern 604, it is difficult to achieve a high level of accuracy in the manufactured product or the micromirror element X6, in terms of relative positions between the comb-teeth electrodes 61a and 62a as well as relative positions between the comb-teeth electrodes 61b and 62b. In other words, according to the above-described convention, it is difficult to form the comb-teeth electrodes 61a, 62a at a high level of alignment accuracy, and it is difficult to form the comb-teeth electrodes 61b, 62b at a high level of alignment accuracy. If the comb-teeth electrodes 61a, 62a do not have sufficient alignment accuracy, when driving the element by applying a predetermined voltage to the comb-teeth electrodes 61a, 62a, an undesirable situation called pull-in phenomenon can develop in which the comb-teeth electrodes 61a, 62a come to contact with each other as a result of mutual attraction. Likewise, if the comb-teeth electrodes 61b, 62b do not have sufficient alignment accuracy, when driving the element by applying a predetermined voltage to the comb-teeth electrodes 61b, 62b, an undesirable situation called pull-in phenomenon can develop in which the comb-teeth electrodes 61b, 62b come to contact with each other as a result of mutual attraction. The pull-in phenomenon must be avoided since it is a hindrance to the element's oscillating drive and oscillating operation.